You can obtain this one as soon as you gonna enter the Biomedical module.
In there you need to go in to "Medical Wing", then straight to Exam Facilities and then a little bit left into Medical Office. Behind a desk, in a drawer, you'll find a "Storage Room Key". Take it, leave Exam Facilities and go right into a Medical storage room. Left from an entrance you'll find a door with a keyhole for your key. Open it and take skull (it's behind some boxes in front of you). Then go back to Exam Facilities and open closet in a furthest right corner. Put the skull on a skeleton.
After putting the skull on a skeleton, go back to a Medical storage room and take a hat (it's gonna be on a table to your right or somewhere near this table).
All is left is just to put a hat on the head of a skeleton.
Unfortunately, there is a common problem with a spawn of this hat that you need for an achievement. It's definitely a bug and there is like 50/50 chance that you gonna have this hat on your playthrough.
The only one thing that I can recommend and which is kinda work - is just to replay this game until hat is gonna be in a Storage room somewhere around the table. I found this hat on a first playthrough but it wasn't there on my second... aaand it was there on my third.
So, good luck with spawning chances.